This invention relates to methods and apparatus for position of one member relative to another member. The position encoder may be a switch which is set in positions respectively representing different values for the derivation of electrical signals representative of those values. For example the electrical signals may represent decimal values in binary coded form.
British patent specification No. 2034991 discloses a ten position switch arranged to generate a 2 out of 5 binary code. This switch consists of five stationery conductive contact strips each consisting of interconnected inner and outer arcuate parts and two common conductor rings, the inner and outer arcuate parts and the common conductor rings being concentric with one another. A rotary part of the switch carries two bridging contacts. One of these contacts bridges between one common conductor ring and the inner arcuate parts and the other bridging contact bridges between the other common conductor ring and the outer arcuate parts. The arcuate contact parts are so arranged relative to one another that a unique 2 out of 5 binary code can be derived from the switch for each of ten positions of the switch.
Switches as described above may be utilised as sensors to derive signals representing the angular position of a movable member by mechanically connecting the member to the rotary part of the switch. Thus as the member moves from one to an adjacent position, the coded signal derived from the switch changes to that 2 out of 5 code which represents the new position of the member. However it will be appreciated that, in order to ensure reliable operation of the switch, the arcuate parts have substantial angular extent and in consequence the coded signal remains unaltered over a substantial angular movement of the member. Hence the coded signal only provides an indication that the member is positioned within a range of positions. While this may be satisfactory for many purposes it is sometimes necessary to provide an indication that a movable member is positioned not only within a selected range of positions but also that it is positioned at a precise selected position within that range.
In postal franking machines it is essential that the values of franking entered into the machine and registered in the electronic accounting portion of the machine are precisely the same at all times as those values which are printed on the mail item. The above described encoding switch is particularly suitable for use in sensing the setting of a printing element which is to be used in printing the franking value because any malfunction of the switch results in derivation of a binary code which is not recognised by the processing apparatus of the machine and therefore an error or malfunction indication is generated and operation of the printing is inhibited. However as mentioned above the switch provides a binary coded signal which indicates only to the extent that the rotary part of the switch lies within a range of positions. This is sufficient where the printing element is set manually by a thumb wheel provided with detents to ensure that the printing element is set to the precise position for printing the required value. If the detents are not provided, the printing element might be slightly offset from its correct position thereby resulting in defective printing of the selected value even though the switch would provide a correct indication of the value being printed.